Saturated Zone drainage + Multi-cell Overland Flow

The topography is often used to define the SZ drainage network. Thus, a refined topography more accurately reflects the SZ drainage network.

The SZ drainage function uses a drain level and drain time constant. The drain level defines the depth at which the water starts to drain. Typically, this is set to some value below the topography to represent the depth of surface drainage features below the average topography. This depth should probably be much smaller if the topography is more finely defined in the sub-grid model. The drain time constant reflects the density of the drainage network. If there are a lot of drainage features in a cell then the time constant is higher and vice versa.

When using the multi-cell OL, the drainage system is updated in the sense that the drain level will be defined using the sub-scale topography informa­tion. The SZ drainage will include the following when using sub-scale:

·         Multi-scale SZ drainage supported only in the PCG transient SZ solver

·         Each sub-grid cell will have the same drain time constant defined by the value in the coarse grid.

·         If the drain level is defined as an elevation, then all sub-grids will have the same drain level.

·         If the drain level is defined by depth below the surface, then each sub-grid may have a unique drain level, since each sub-grid can have a dif­ferent elevation. Each coarse grid cell has a water table that is common for all fine scale grids within the coarse grid.

·         If the coarse cell water table is above the fine scale drain level, then drainage is calculated based on the drain time constant and the depth of water above the fine scale drain level.

·         Total drainage in a coarse cell is the sum of all the fine scale drainage volumes.

·         Drainage routing by levels will be determined by the coarse grid. How­ever to make it more realistic with respect to the fine scale hydrology, the drainage routing by levels will be based on the lowest drain level in a coarse cell.

·         Drainage to local depressions will be added to the SZ cell, and resultant ponding will then follow the multi-scale OL flow.

Disabling Multi-Cell Drainage

By default, if the multi-cell OL option is invoked, multi-cell drainage will be active. If you want to disable multi-cell drainage, perhaps for backwards com­patability with older models, an Extra parameter option is availble to switch off multi-cell drainage: .

If this option is used, then the multi-cell drainage is switched off and the drain­age will function using the groundwater level and drain level based on the course cells.